If you've heard any of the noise surrounding Google's supplemental results feature, you could be forgiven for thinking that some sort of plague had hit the SEO community. "But my site has lots of quality content! Those are my main information pages! Why are they being sent to supplemental?"
This isn't exactly a new issue. Google introduced their "supplemental result" directory in 2003. It serves as sort of a "last chance" reference for more obscure searches. If you search for something and Google can't find anything good in their main index, they'll dip into the supplementary index to see if there's anything there that's suitable. But because these pages have been deemed lower quality, less relevant, repetitive, (insert other theorized measure here) they don't appear in the main index. Unless you're banking on some pretty obscure terms for your traffic, once your pages disappear into the supplemental index your traffic for those search terms drops down to nil. It also means that Google crawls your pages less frequently and may not notice when you undertake dramatic measures to resurrect your pages.
People who aren't watching their search engine listings like a hawk might not even notice when it happens to their pages. The people who cry the loudest are those who were previously receiving a lot of traffic to their site through particular pages, and who notice a dramatic drop in traffic once those pages disappear into the supplemental index. If you haven't seen it already, here's what it looks like when your pages are sent to "Google hell":
So why might a page that has previously been receiving some Google lovin' suddenly drop out of the search engines? And not just drop down a couple of pages, but completely disappear from the main index? Theories have included:
* Duplicate content
* Exchanging links with irrelevant websites
* Excessive link exchanges
Matt Cutts (your friendly neighbourhood Googleworker) points to pagerank as being the main culprit . He suggests that if your pages have suddenly been dropped into the supplemental index, it might be an indicator that your pagerank has slipped a little way. This doesn't necessarily mean the other theories are incorrect... it's just that these other factors influence your pagerank one way or another.
* Exchanging links with irrelevant websites: It's long been hinted that the relevancy of a link partner influences how much benefit you receive from their link. If you've got links from irrelevant websites, they might stop passing on as much pagerank as previously. If you're linking to irrelevant websites, it might hurt your pagerank a little more.
* Excessive link exchanges: It's also been hinted that Google will start looking harder at sites that have an "excessive" number of link exchanges, and that perhaps these links will stop passing on as much pagerank. So if you've got a rather large number of link exchanges going and your pages drop into supplemental, it might be because you're not receiving any goodness from those links anymore.
* Duplicate content might not be bad in itself, but it might mean that you're trying to share pagerank over multiple pages. So long as there was a reasonable amount of pagerank flowing to both of the pages, they should be reasonably safe.
As is the case with most of Google's algorithms, I'm sure the supplemental issue is anything but clear-cut, but the message seems to be "keep the pagerank flowing and your pages will stay out of supplemental". And if you've got pages that have "gone supplemental" already (which is only really a bad thing if you're desperately wanting those pages to be in the running for search terms in the main index) then to get them out would be a case of improving the flow of pagerank through your site or getting more relevant links to those pages.
Be warned... though... it might take a while
Take care and have a good weekend everyone.